Under the "Offline Domain Join" option for an "Apply System Configuration" task, the "Select a domain to join" drop-down is blank. No domain is available.
ITMS 8.7.1
This is expected. The reason why there is no a domain to choose from is that the "offline domain join" information has not been imported yet.
This "Offline domain join" feature is new with our ITMS 8.7.1 release. See release notes.